1. Field of the Invention
The invention relates to an electric power steering system that assists a vehicle steering operation.
2. Description of the Related Art
A conventional electric power steering system that assists a vehicle steering operation is described in Japanese Patent Application Publication No. 2011-203091 (JP 2011-203091 A). The electric power steering system includes a torque sensor and a controller that controls driving of a motor. The torque sensor includes a Hall IC and a magnetic circuit that applies magnetic flux corresponding to steering torque applied by a driver to the Hall IC. In the electric power steering system, when the steering torque varies in response to a driver's steering operation, the magnetic flux applied from the magnetic circuit to the Hall IC of the torque sensor varies. Thus, a detection signal corresponding to the steering torque is output from the torque sensor. The controller computes the steering torque on the basis of the detection signal from the torque sensor, and computes an assist command value on the basis of the detected steering torque value. Then, the controller controls driving of the motor to cause a torque output from the motor to follow the assist command value.
The electric power steering system described in JP 2011-203091 A includes a magnetic field generator that periodically applies a magnetic field to the torque sensor, aside from the magnetic circuit. When a magnetic field is applied to the torque sensor through the use of the magnetic field generator, the controller determines whether a signal based on the applied magnetic field is output from the torque sensor. When the signal based on the applied magnetic field is not output from the torque sensor, the controller determines that a malfunction has occurred in the torque sensor. The controller detects the steering torque on the basis of the detection signal from the torque sensor immediately before the magnetic field is applied to the torque sensor from the magnetic field generator, and the controller holds the detected steering torque value during a period in which the magnetic field is generated from the magnetic field generator.
In some electric power steering systems, a compensation value is computed based on, for example, a torque differential value, which is a first-order time differential value of the detected steering torque value, or an angular velocity of a motor, and compensation is provided to an assist command value based on the computed compensation value in order to improve the stability of a control system and the steering feel (refer to, for example, Japanese Patent Application Publication No. 2006-131191 (JP 2006-131191 A) and Japanese Patent Application Publication No. 2009-269540 (JP 2009-269540 A)).
As in the electric power steering system described in JP 2011-203091 A, when periodic updating and holding of the detected steering torque value are alternately executed, the characteristics of the control system are switched between the state where the detected steering torque value is periodically updated and the state where the steering torque is held, and thus the control system is likely to be unstable. If the control system is unstable, for example, vibrations are generated in a steering mechanism due to the assist torque. This may give an uncomfortable feeling to the driver.